So one of the things I've done to get interaction with multiple worlds is I've been quickly changing the position of player before running interactions on other worlds.

 

Using a rotation matrix I've been able to transform the player pos before things like block placement, tile entities, etc.

 

I know this is terrible on client side because the rendering thread can start a frame between these translations, but its fine on server right? I havent seen any problems with it so far, but would the net thread be sending packets on a different schedule than a tick (ie would the net thread send player position between ticks or only before/after )

More or less single-threaded*

 

In this case, yes, only one thread is involved.  The way Minecraft is multithreaded right now is limited to chunk loading/unloading, although I believe I found another place recently involving sounds.  All I know is that it is probably my code and that whatever it is causes a concurrent modification on the

tickingSounds

list.

And networking also runs on separate thread now.